1. Field of the Invention
This invention relates generally to marine seismic exploration, and, more particularly, to a steerable hydrofoil for use in marine seismic exploration.
2. Description of the Related Art
Marine seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits because many hydrocarbon deposits are found beneath bodies of water. FIG. 1 conceptually illustrates a first embodiment of a conventional system 100 for carrying out a marine seismic survey. In the illustrated embodiment, one half of the conventional system 100 is shown, but it should be understood that the conventional system includes a second half above the symmetry line 102. The conventional system 100 includes a survey vessel 105 coupled to a seismic array 110, which typically includes one or more streamers 115.
The streamers 115 include passive streamer sections 117, sometimes referred to as stretches, which may be used to dampen vibrations. The passive streamer sections 117 may have a length that ranges from about 50 to 150 meters. For example, the passive streamer sections 117 may have a length between 75 and 100 meters. Typically, the streamers 115 and, if present, the passive streamer sections 117, are coupled to the survey vessel 110 by lead-in cables 120. Separation ropes 123 may also be provided to spread out the streamers 115 and, if present, the passive streamer sections 117. One or more seismic sensors 125, such as hydrophones, may be distributed along the length of the seismic streamer 115. Although not shown in FIG. 1, one or more seismic sources may also be included within the conventional system 100.
In operation, the survey vessel 105 attempts to tow the seismic array 110 along a predetermined path. As the seismic array 110 passes over a selected portion of the sea floor beneath the predetermined path, the seismic sources may be used to drive an acoustic wave, commonly referred to as a “shot,” through the overlying water and into the ground. The acoustic wave is reflected by subterranean geologic formations and propagates back to the seismic sensors 125. The seismic sensors 125 receive the reflected waves, which are then processed to generate seismic data. Analysis of the seismic data may indicate probable locations of geological formations, such as hydrocarbon deposits, that may exist beneath the sea floor along the predetermined path.
The accuracy of the seismic survey is determined, in part, by how accurately the seismic array 110 is towed along the predetermined path. Thus, in addition to guiding the seismic array 110 by steering the survey vessel 105, the marine seismic surveying system 100 includes hydrofoils 130 coupled to the seismic array 110. For example, Western Geco Monowings® may be coupled to one or more of the lead-in cables 120 and/or the passive streamer sections 117 of the seismic array 110. Although two hydrofoils 130 are shown in the embodiment illustrated in FIG. 1, persons of ordinary skill in the art will appreciate that more or fewer hydrofoils 130 may be coupled to the seismic array 110. Moreover, in some alternative embodiments, the hydrofoils 130 are towed by a separate rope or wire, and are coupled to the seismic array 110 by strong separation ropes that are usually called lever arms (not shown). In these cases, hydrofoils 130 of a type usually referred to as a door, paravane, or Barovane, are typically used.
To provide sufficient lift to steer the front end of the seismic array 110 and/or to maintain a spread of the seismic cables 115 and/or the passive streamer sections 117, a typical hydrofoil 130 used in a marine seismic survey is approximately 7-10 meters tall and has a 1-2 meter chord length. In accordance with common usage in the art, the chord length of the hydrofoil 130 is defined herein as the distance from the nose to the tail of the hydrofoil 130. A hydrofoil 130 of this size may have a lift of about 10 tons.
Conventional hydrofoils 130 are typically steered passively to a desired mean position along the predetermined path, at least in part because the power required for active continuous steering of the large hydrofoils 130 is relatively large and not generally available. Passive steering of the hydrofoils 130 is typically capable of steering the seismic array 110 through a range of about 500-600 meters in the cross-line and/or in-line directions. However, variable water currents and the like along the predetermined path may cause the hydrofoil 130 to deviate from its desired mean position. Consequently, the front end of the seismic array 110 and/or the location of one or more of the streamers 115 may also deviate from their desired positions. For example, the seismic array 110 and/or the streamers 115 may deviate from their desired positions by a positioning error of about ±5-10 meters. The deviations of the seismic array 110 and/or the streamers 115 may be in either the cross-line or the in-line direction. Alternatively, when the seismic array 110 is steered to repeat the path of a previous seismic survey, then the desired path of travel may not be a straight line. Deviations from this line may cause cross-line position errors.
The positioning errors caused by the deviations of the seismic array 110 and/or the streamers 115 introduce noise into the seismic data. For example, the positioning errors may degrade the time-lapse signal-to-noise ratio of the seismic data. The positioning errors may also propagate from a front end to a back end of the seismic array 110 and/or the streamers 115 and, depending on factors such as the water currents, the positioning errors may increase from the front end to the back end of the seismic array 110 and/or the streamers 115. Furthermore, the positioning errors may propagate from one survey to another when seismic data is collected in multiple surveys that are repeated over a period of time and then combined, or stacked, to form a combined seismic data set.
Conventional hydrofoils 130, such as doors, paravanes, Barovanes, and the like are not typically used to correct for path deviations, such as those caused by current variations. For example, conventional hydrofoils 130 are typically used near their maximum lift capacity in a standard efficient tow configuration, such as shown in FIG. 1, which may limit the ability of the hydrofoil 130 to compensate for path deviations. Although the towing configuration of the one or more hydrofoils 130 may be changed so that the hydrofoils 130 operate at lower lift powers, e.g. approximately 65% of their maximum lift power, this approach would provide a less efficient configuration with longer lead-in cables 120, reduced efficiency in terms of reduced maximum spread, longer lay backs resulting in difficulties in re-positioning by vessel steering, deep cables, and other undesirable consequences. Moreover, cross-line steering of the hydrofoil 130 may introduce undesirable changes in the in-line position of the streamers 120.
FIG. 2 conceptually illustrates movement of the hydrofoil 130 described above, such as a door, a paravane, a Barovane, and the like. As an angular deviation 205 of the hydrofoil 130 increases in the direction indicated by the arrow, a drag 210 of the hydrofoil 130 and a lead-in tension 215 increase correspondingly. Consequently, a lift 220 needed to oppose the drag 210 and the lead-in tension 215 increases significantly. Achieving the required lift 220 may require increasing an angle of attack of the hydrofoil 130 into a range in which the hydrofoil 130 may stall and/or become unstable. These disadvantages may also limit the ability of the hydrofoil 130 to compensate for path deviations.
Referring back to FIG. 1, the hydrofoil 130 also creates a wake 135 of highly rotational fluid. Since the seismic array 110, the streamers 115, and the sensors 125 are towed approximately behind the hydrofoil 130, the wake 135 often disturbs the seismic array 110, the streamers 115 and/or the seismic sensors 125. Wake disturbances add noise to the seismic data. Moreover, the wake noise introduced by wake 135 of the hydrofoil 130 may be increased if the hydrofoil 130 is steered. A non-steerable, fixed angle-of-attack hydrofoil (not shown), such as Western Geco's non-steerable Miniwing® may be coupled to the front of one or more of the streamers 115 to pull the streamer 115 about 15-20 meter out of the wake 135. However, the angle-of-attack of the non-steerable, fixed angle-of-attack hydrofoil may not be changed during a survey to account for changing conditions.
One or more birds 140 may also be attached to the streamers 120. A typical bird 140 has a combined wing span of about 1 meter and has a chord length of approximately 20 centimeters. The birds 140 provide force cross-line to the streamers 115 and are typically used for depth keeping and to compensate for variable current conditions. Conventional birds are only capable of providing forces in the vertical plane for depth keeping purposes. However WesternGeco birds, called Q-fins®, are capable of providing cross line forces in both the vertical plane, for depth keeping, and in the horizontal plane. The latter is used for keeping a straight streamer in spite of varying currents, keeping constant streamer separation and to steer sideways in order to achieve a given demanded feather. The birds 140 may also be steerable. However, due at least in part to high tension in the streamers 115, the passive streamer sections 117, the stretches 123, and the lead-in cables 120, the steerable birds 140 are typically not powerful enough, i.e. they do not provide sufficient lift, to help position the front end of the streamers 115 and/or the array 110. For example, several hundred meters and several steerable birds 140 may be required to achieve a desired position for the streamers 115 and/or the array 110. Moreover, such hard steering of the steerable birds 140 may also increase noise in the seismic data and limit the steerable birds 140 ability to compensate for varying current conditions and/or to steer the seismic array 110 out of the wake 135 of the hydrofoil 130.
In summary, due in part to constraints such as cost, power consumption, noise levels, and desired function of existing elements, the conventional marine seismic survey system 100 lacks a mechanism for maneuvering the front end of the seismic array 110 and/or streamers 115 within a relatively small range of ±20 meters in the cross-line direction. The conventional marine seismic survey system 100 also lacks a mechanism for reliably positioning the front end of the seismic array 110 and/or streamers 120 with an error of less than or about ±1 meter. Consequently, undesirable noise, e.g. noise from excess steering of the hydrofoils 135 and/or the steerable birds 140, noise from positioning errors, and/or noise from the wake 135 of the hydrofoil 130, may be introduced into seismic data collected by the conventional marine seismic survey system 100.
The present invention is intended to address one or more of the problems discussed above.